1. Field of the Invention
The invention relates to a perpetual calendar, and more specifically to a perpetual calendar which accommodates permanent notes for birthdays and other anniversaries on a conventional full-month display.
2. Description of Related Art
Many perpetual calendars have been developed which display the days of each month in seven columns corresponding to the days of a week. They all depend on the finite number of combinations that occur; a month can only start on one of seven days and have one of four lengths: 28, 29, 30 or 31 days. Many of the existing designs employ lists or look-up tables to identify which of 7 to 28 displays to use for a given month and year. Others have a date panel with a redundantly extended array, in which dates are repeated in thirteen or more columns, behind a mask which is aligned to show the correct seven columns. These "slide calendars" may be set to a code from a look-up table or by alignment of month and year on scales which appear on the mask and movable date panel.
The mathematics behind look-up tables or slide calendar scales are straight forward: A normal year of 365 days contains 52 weeks plus one day. Consequently each month will begin one day later in the following year. In leap years (years evenly divisible by 4) February has 29 days, causing March and subsequent months to start two days later than in the previous year. A 28-year repeat cycle occurs; 21 regular years at 1 day plus 7 leap years at 2 days equals 35 days or 7 weeks. Because years divisible by 100 are not leap years unless they are divisible by 400, three out of four centuries have 76 regular years and 24 leap years; 76+24.times.2=124 days, 2 days short of 18 weeks. Thus each century the first-day-of-the-month pattern moves two days earlier. The one-day shift for the leap century brings the total to 7 days, making each of these four-century periods identical.